This invention relates to apparatus for measuring the diffusion capacity of a barrier or the diffusive flux through a barrier and a method for calibrating the apparatus.
Cigars, cigarettes, and cigarillos (generally known as smoking articles) typically comprise tobacco cylindrically surrounded by a paper wrapper. In use, a smoking article has a burning coal at one end, and a supply of oxygen is needed to feed the hot coal. Some oxygen is supplied directly to the hot coal from the atmosphere and some oxygen is supplied to the hot coal through the wrapping paper by diffusion. Some combustion products can also leave the region of the hot coal through the wrapping paper.
Cigarette wrapping paper is generally permeable to air by design. There are many known methods for determining paper permeability. These may involve applying a pressure differential across the paper and measuring effects such as gas flow rates that arise by result.
Techniques for determining paper permeability using pressure differential may be effective for determining the gas flow rate across the paper during puffing of a cigarette (where there is also a pressure differential across the paper). These techniques may be less effective for determining the gas flow rate across the paper during the smoulder phase of a cigarette where there is a minimal pressure differential across the paper.
In addition, while techniques for measuring paper permeability using pressure differentials may be sufficiently accurate when the permeability of the cigarette wrapping paper is relatively high, these techniques may lack accuracy in situations where the permeability of the paper is relatively low. One example where this may occur is in the measurement of the permeability of paper used in fire standard compliant smoking articles.
Fire standard compliant smoking articles use wrapping paper with annular bands of different permeability. Extended bands of standard permeability are punctuated by thin bands of low permeability. A burning cigarette may be extinguished when the combustion reaches bands of low permeability because (in the absence of a puffing action) insufficient oxygen may be supplied to the burning coal by diffusion through the wrapping paper.
Other methods for measuring the propensity of gas to be transported across a paper barrier involve measurements of the diffusion capacity of the barrier. Diffusion capacity is analogous to permeability and is measured in the same units. In these methods, a chamber is separated into two regions by a paper barrier, and a pressure differential of zero is applied across the barrier. Different types of gas are provided on either sides of the paper barrier, and gas concentration meters can measure the rate of diffusion through the paper. These methods have the advantage of being representative of behaviours present during smouldering. One apparatus for measuring the diffusion of gas through cigarette wrapping paper is described in US 2005/0087202.
A problem with standard apparatus that measure diffusion capacity, or that measure the diffusive flux of gases across a barrier, is in the verification of the accuracy and stability of the measurements. Known apparatus may be able to determine a difference between the diffusion capacities of different samples, but may lack the ability to provide a reliable absolute measure of diffusion capacity or diffusive flux.
The present invention addresses this problem and aims to provide apparatus and methods for use in calibrating apparatus in the measurement of diffusive flux.
According to an aspect of the present invention there is provided measurement apparatus comprising: a chamber; a barrier separating the chamber into first and second regions, wherein the barrier comprises one or more through-holes each with known properties; a detector arranged to measure a parameter that is related to the diffusive flux of a gas from one region to the other, through the barrier; a processor to connect a parameter derived from the measurement and a parameter derived from the known properties of the through-holes; and data storage means for storing the calibration parameter calculated by the processor, wherein the barrier is replaceable by a further barrier, such that the apparatus is able to determine a parameter that is related to the diffusive flux across the further barrier by measuring a parameter with the detector and applying the calibration parameter stored in the data storage means.
In this way the apparatus can accurately determine the diffusion capacity of a barrier. This may be achieved by measuring a parameter that is related to the diffusive flux of gas from one region to the other, through the barrier, applying the calibration parameter, and calculating the diffusion capacity based on the calibrated measurement. The actual measurement made by the detector may be the concentration of gas that has diffused from one region of the chamber to the other. This measurement is, of course, related to the diffusive flux from one region to the other, through the barrier. In other embodiments the detector could measure other parameters such as gas flow rates through the barrier as these may also be related to the diffusive flux of gas from one region to the other.
Calibration of the apparatus may be important because measurements of diffusion capacity may be inaccurate otherwise. This may be of particular benefit in the manufacture of cigarette papers where fine measurements of the diffusion capacity of paper are required.
In the industry it is common to refer to the diffusivity or the diffusion capacity of a barrier in order to refer to the propensity of the barrier to allow diffusion. The diffusion capacity is often denoted as D* and is measured in units of ms−1 which are the same units as are often used to measure permeability.
Preferably the apparatus comprises a calibration coefficient pre-stored in the data storage means for converting a measured parameter into a calibrated parameter related to the diffusive flux of a gas across the barrier. The calibration parameter calculated by the processor may replace this pre-stored calibration coefficient, or it may be used as a corrective factor.
The calibration parameter may connect a variety of different parameters. For example, the processor may be arranged to calculate a parameter that is related to the theoretical diffusive flux through the barrier, based on the known properties of the through-holes, and the calculated calibration parameter may connect the measured parameter and the calculated parameter. Thus, the calibration parameter may connect the actual concentration of gas that is measured at the detector with the theoretical concentration of gas that would be expected. In an alternative the processor may calculate a parameter from the measured parameter that is related to expected properties of the through-holes. In this example the calibration parameter may connect the known properties of the through-holes with the expected properties based on the measurement of the detector.
Preferably said known properties of the through-holes include the depth and area of the through-holes. In this way the theoretical diffusive flux across the barrier may be calculable from first principles according to Fick's laws of diffusion. The theoretical diffusive flux across the barrier may be converted into another parameter such as the theoretical concentration of gas that is expected for detection by the detector.
The known area of each hole may be based on the diameter or the radius of each hole. Thus, the known properties of the through-holes may include the depth and diameter of the through-holes. The through-holes may take any shape, but it may be most convenient to provide a circular shape.
It may be desirable to provide a barrier with a low diffusion capacity when calibrating the apparatus. This may be desirable when the apparatus is intended to be used to measure the diffusive flux across materials such as some cigarette papers, as these are known also to have a low diffusion capacity. A greater accuracy can be achieved by calibrating the apparatus with a barrier having a diffusion capacity that is as similar as possible to the diffusion capacity of those barriers that will subsequently be analysed.
A barrier with a low diffusion capacity may comprise through-holes with a large depth, and/or a small number of through-holes, and/or through-holes with a small area. It may be particularly convenient to provide a barrier with a low diffusion capacity by providing through-holes that have a depth that is greater than the thickness of the barrier. This may be achieved by fixing a tube to the barrier. All of the through-holes in the barrier may be defined at least in part by tubes in this way.
The at least one through-hole with a depth that is greater than the thickness of the barrier may be defined at least in part by a tube that is fixed to the barrier. It may be particularly desirable to define a through-hole with a tube because the tube can be manufactured to have well defined properties. For example, the tube may be a regular cylinder with well defined, smooth openings. In this way, predictable effects may occur when air is drawn through the tube by diffusion. Thus, any errors in the calculated parameter related to the theoretical diffusive flux across the barrier that are due to turbulent airflow in the through-holes may be reduced. The ratio of hole area or circumference to depth may be chosen to ensure diffusive flow with a low pressure drop across the barrier.
The tube may extend from a surface of the barrier, and the tube may be fused or fixed to the barrier. The tube may also extend through the barrier such that it projects from opposing surfaces thereof. Preferably the tube is of glass or ceramic material.
At least one of the through-holes in the barrier may have a depth equal to the thickness of the barrier. In this way, the through-hole may be a simple hole in the material of the barrier. This may be desirable due to ease of manufacture. The through-holes may be drilled in the barrier by any convenient means such as by a laser.
Preferably the barrier is of a material that is substantially impermeable to air in the absence of any through-holes. Thus, any diffusion across the barrier may be due to diffusion through the known through-holes only. The barrier may be made of one of metal, glass, ceramic and polymer material. Preferably the barrier is made of metal foil which may be thin and flat.
The material of the barrier may have a low coefficient of linear expansion. In this way, the properties of the through-holes may be substantially unaffected by small changes in temperature so that the diffusion capacity of the barrier is substantially independent of small changes in temperature. Another desirable property of the material chosen may be its propensity to be drilled with holes using a laser or similar device.
A suitable choice of material for the barrier can allow simple cleaning of the barrier. For example, the barrier may be cleaned by an ultrasonic cleaning bath with a suitable solvent or by heating. Regular cleaning of the barrier may be desirable to avoid any clogging of the through-holes such as may occur through atmospheric contamination. This can avoid any error between a calculated theoretical diffusive flux across a barrier, and an actual diffusive flux which is lower because of clogged through-holes.
Preferably the barrier is clamped when it is held to separate the chamber into first and second regions. The material of the barrier is preferably chosen so that there is no deformation of the barrier due to clamping as such deformation may affect the diffusive flow across the barrier.
Preferably the detector is provided in the first region and the detector is arranged to measure the concentration of a gas in the first region, at least some of the gas having diffused into the first region from the second region, through the barrier. Thus, the concentration of the gas as detected in the first region may be dependent on the rate of diffusion of gases from the first region to the second region.
The diffusivity measurement apparatus may comprise a further detector provided in the second region, arranged to measure the concentration of a gas in the second region at least in part due to the diffusion of the gas from the first region, through the barrier. Thus, the apparatus may be calibrated by measurements from the detector in the first and/or second region. It may be possible for the apparatus to be calibrated based on measurements from one region and for the diffusion capacity of further barriers to be determined according to measurements from the other region.
According to another aspect of the present invention there is provided a method of calibrating measurement apparatus, the method comprising the steps of: separating a chamber into first and second regions with a barrier, wherein the barrier comprises one or more through-holes each with known properties; measuring a parameter that is related to the diffusive flux of gas from one region to the other, through the barrier; calculating a calibration parameter to connect a parameter derived from the measurement and a parameter derived from the known properties of the through-holes; and recording the calibration parameter, such that the apparatus is able to determine a parameter that is related to the diffusive flux across a further barrier by measuring a parameter and applying the recorded calibration parameter.
Preferably the method comprises the further step of calculating a parameter that is related to the theoretical diffusive flux through the barrier, wherein the calculation is based on the known properties of the through-holes, and wherein the calculated calibration parameter connects the calculated parameter and the measured parameter.
Preferably the properties of the one or more through-holes (such as the depths of the through-holes and the area of the through-holes) are known because they have been measured. The measurement of the properties of the one or more through-holes may be undertaken optically or using any other standard techniques so that they are known to a high degree of accuracy.
According to yet another aspect of the present invention there is provided a method of determining a calibrated diffusive flux of gas through paper for a smoking article comprising the steps of calibrating diffusivity measurement apparatus according to the method as previously defined; replacing the barrier with a further barrier to separate the chamber into first and second regions; measuring a parameter that is related to the diffusive flux of gas from one region to the other, through the barrier; and calculating a calibrated diffusive flux at least in part by applying the recorded calibration parameter to the measured parameter.
In this way the diffusive flux across smoking article paper (such as the paper for fire standard compliant cigarettes) can be accurately determined because the measurement apparatus has been properly calibrated. The diffusion capacity of smoking article paper and other related properties may also be determined in this way.
The chamber of the apparatus may include means for active management of the pressure differential across the barrier in order to maintain a minimal pressure differential. The pressure differential is preferably maintained at a level that is low enough so that the diffusive flux is greater that the pressure drive flow. Ideally the ratio of diffusive flux to pressure driven flow should be greater than 100:1. Depending upon the nature of the barrier the desirable pressure difference may be as low as 0.01 to 0.1 Pa.